The invention relates to a method of manufacturing mono-layer capacitors having a ferroelectric layer on the basis of titanate as a dielectric on a substrate, the ferroelectric layer being located between a first and a second electrode of noble metal.
Ferroelectric ceramic material containing as main constituent, for example, BaTiO.sub.3 is used as a dielectric of capacitors on account of its high dielectric constant.
Capacitors comprising ferroelectric materials, such as, for example, BaTiO.sub.3, require for a plurality of applications a low dependence of the capacitance upon the temperature. By way of example, capacitors having an X7R-characteristic are mentioned, in which .DELTA.C/C (25.degree. C.)=.+-.15% lies in the range of from -55.degree. C. and +125.degree. C. For this purpose, specific dopants, such as, for example, Nb and Co, must be introduced into the barium titanate. Such capacitors are manufactured nowadays in a multi-step process, BaTiO.sub.3 powder having the desired doping being mixed with a binder and being pulled out to foils. These foils are alternately stacked between electrodes, which consist of Pd, Pd/Ag alloys or Ni on account of the high sintering temperature of the ceramic material of BaTiO.sub.3. The stacked foils are sintered at a temperature of 1300.degree. C. After this process, multi-layer capacitors are manufactured, in which the thicknesses of the ceramic dielectric lie between 15 and 30 .mu.m and which due to the stacking of up to 50 ceramic layers having surface areas of 1 to 7 mm.sup.2 have capacitances of up to 500 nF. This means that each ceramic layer has a sheet capacitance of 2 to 3 nF/mm.sup.2. In order to obtain in this known process capacitors having high capacitances, the grain size of the ceramic material of BaTiO.sub.3 must be adjusted optimally. With a grain size of .apprxeq.0.7 .mu.m it is possible to obtain ceramic materials of BaTiO.sub.3 having a dielectric constant .epsilon. of 3000 to 4000. With such a grain size of the ceramic material, comparatively high insulation resistances are obtained in the ceramic layers having layer thicknesses of 15 to 30 .mu.m; with 15 .mu.m thick Nb/Co-doped ceramic layers of BaTiO.sub.3 insulation resistances of 10.sup.9 to 10.sup.10 Ohm.m were measured at 5 V/.mu.m and 20.degree. C.
In view of the miniaturization of electronic components, it is desirable to increase the volume capacitance of capacitors. This can be attained by reduction of the thickness of the dielectric layers. Further, the direct integration of a capacitor into integrated circuits comprising silicon single crystal wafers is desirable because this would lead to a substantial reduction of the cost for the installation of the components and to an improvement of the electrical properties due to the line inductances and line capacitances connected to the supply conductors becoming superfluous. This is of particular advantage with the use of the components in electronic computers.
The main obstacle in reducing the dielectric layer thickness d to values below 15 .mu.m is the strong decrease of the insulation resistance with the use of conventional ceramic powders, such as are used for the manufacture of ceramic foils. For example, with Nb, Co-doped ceramic layers of BaTiO.sub.3 having a thickness of about 5 .mu.m, resistivities of 10.sup.8 Ohm.m were measured. With a constant voltage U (for example 5 V), the decrease of the resistivity is even more strongly pronounced. Essentially the small number of grains between the electrodes leads to the small resistances of thin ceramic layers of BaTiO.sub.3. It is known that the insulation resistance in ceramic material of BaTiO.sub.3 is mainly determined by the very high-ohmic grain boundaries. If now the layer thickness is reduced, while the grain size remains unchanged, of course the number of grain boundaries between the electrodes decreases. This adversely affects the insulation resistance of the layer.
The life of the components, which is determined in the first instance by degradation induced by direct voltage, is also determined by the grain boundaries; the life decreases overproportionally with increasing field strength (by reduction of the layer thickness d with the same maximum nominal voltage U).
Attempts have been made to eliminate both the problem of the small insulation resistance and the problem of the reduced life of thin ceramic layers of BaTiO.sub.3 by a reduction of the grain size in these layers.
It is known, for example, from J. Appl. Phys. 55 (1984), p. 3706 ff, to use cathode sputtering methods for depositing barium titanate layers. The cathode sputtering method has the disadvantage, however, that on the one hand it is technically very complicated, while on the other hand the envisaged deposition of multi-component layers having the desired stoichiometry is possible only with great difficulty. Moreover, reaction temperatures in the range of from 900.degree. to 1200.degree. C. are necessary to manufacture BaTiO.sub.3 layers having high capacitances. These reaction temperatures are too high for the integration of a capacitor into a silicon single crystal wafer. According to the known method, 2.5 .mu.m thick BaTiO.sub.3 layers were deposited on Pt substrates. For the layers treated at a temperature of 1200.degree. C., a sheet capacitance of 25 nF/mm.sup.2 and an insulation resistivity of 10.sup.7 Ohm.m were measured at 0.4 V/.mu.m.
It is known from U.S. Pat. No. 3,002,861 to use chemical coating methods for manufacturing barium titanate layers, starting from barium and titanate alkoxide solutions. These starting materials have the disadvantage, however, that especially barium alkoxide is very sensitive to hydrolysis and that a reproducible deposition of barium titanate layers is difficult. A further disadvantage is that the layers thus formed are multi-phase layers and contain besides BaTiO.sub.3 also Ba.sub.2 TiO.sub.4 and BaTiO.sub.5.
From Am. Cer. Soc. Vol. 55 (1976), p. 1064 ff. it is known, starting from barium naphthenate and titanium alkoxide solutions, to deposit 1-2 .mu.m thick barium titanate layers on substrates of glass or quartz glass for the use in IR spectral analyses. The layers thermally decomposed at a temperature in the range of from 200.degree. to 800.degree. C. exhibit for layers having layer thicknesses &gt;1 .mu.m a grain size of 0.5 .mu.m. With reference to the examinations which have led to the present invention, it can be ascertained that a grain size of 0.5 .mu.m is too large for a 1-2 .mu.m thick layer because in this case the number of grain boundaries per layer is too small; the values for the insulation resistance then become too low.